Objectives: (1) The question how the interaction with actin differs for the two heads of myosin or HMM is of considerable interest and has been investigated repeatedly with contradictory results. We want to complete our studies showing that the second head of HMM, in the absence of ATP, binds of actin with a larger binding constant than the first head-presumably because the second head is held close to actin by the complex between actin and the first head. (2) We intend to study how the presence of regulatory proteins on actin affects rigor binding in the presence of calcium, following up our observation that regulatory proteins accentuate the advantages of two-headedness. (3) We will continue our studies on the mechanism that makes NEM modified myosin incapable of relaxing in the absence of calcium. (4) We will continue our studies on the existence of ternary complexes between actomyosin and nucleotides.